Vehicle command and control system

ABSTRACT

A dual frequency ranging and communication system for communicating with and locating mobile vehicles in a multipath environment such as an urban center. This system functions in urban areas for both communication and ranging where line of sight radiation techniques may not properly function. A plurality of remote relay stations are located at points throughout the urban area, each station receiving and transmitting coded digital data and tone signals, the tone signals being used for phase ranging. Each of the vehicles is equipped with a transponder responsive only to the radio signal having a formatted message including the vehicle identification code with a ranging tone suitably impressed thereon. The formatted signal is transmitted from one of the relay stations at a first frequency and picked up by a preselected number of other relay stations as well as by the vehicle transponder. The vehicle transponder in turn generates a reply signal at a second frequency containing message data and a tone signal modulated on the second frequency for use in ranging, all of which signals are processed at a central location.

United States Patent 1 Fuller et al.

Jan. 30, 1973 [541 VEHICLE COMMAND AND CONTROL SYSTEM [75] Inventors:Roger L. Fuller, Stow; Sven G. Gustafsson, Framingham; Derek V. Harris,Acton; Robert K. Kaye, Framingham; Joseph J. Oliver, Allston, all ofMass.

[73] Assignee: Raytheon Company, Lexington,

Mass.

221 Filed: July 30, 1970 [21] Appl. No.: 59,504

[52] US. Cl. ..343/6.5 LC, 343/65 SS, 343/15 [51] lnt.Cl ..G0ls 9/56[58] Field of Search ..343/6.5 R, 6.5 LC, 6.5 SS,

[56] References Cited UNITED STATES PATENTS 3,611,379 10/1971 Deckett..343/6.5 LC

3,223,998 12/1965 Hose ..343/6.5 LC

3,474,460 10/1969 Huebscher ..343/6.5 LC

3,343,163 9/1967 Crooks et al ..343/6.S R

RELAY STATION l MASTER MODE RELAY STATION SLAVE MODE CONTROL CENTERPrimary Examiner-Benjamin A. Borchelt Assistant ExaminerN. MoskowitzAttorney-Milton D. Bartlett, Joseph D. Pannone and David M. Warren [57]ABSTRACT A dual frequency ranging and communication-system forcommunicating with and locating mobile vehicles in a multipathenvironment such as an urban center. This system functions in urbanareas for both communication and ranging where line of sight radiationtechniques may not properly function. A plurality of remote relaystations are located at points throughout the urban area, each stationreceiving and transmitting coded digital data and tone signals, the tonesignals being used for phase ranging. Each of the vehicles is equippedwith a transponder responsive only to the radio signal having aformatted message including the vehicle identification code with aranging tone suitably impressed thereon. The formatted signal istransmitted from one of the relay stations at a first frequency andpicked up by a preselected number of other relay stations as well as bythe vehicle transponder. The vehicle transponder in turn generates areply signal at a second frequency containing message data and a tonesignal modulated on the second frequency for use in ranging, all ofwhich signals are processed at a central location.

24 Claims, 5 Drawing Figures RELAY STATION SLAVE MODE RELAY STATIONSLAVE MODE TWO WAY I200 BPS TELEPHONE LINES 20 l 1 l l 1 l 1 l TELEPHONEMODEM BANK PRINTER INPUT/OUTPUT MODEM CONTROLLER COMPUTERPAIENIEIIIIIaII I975 3.714.650

SHEET 10F 3 RELAY STATION SLAVE MODE RELAY STATION MASTER MODE RELAYSTATION SLAVE MODE RELAY STATION SLAVE MODE TWO WAY |2OO BPS TELEPHONELINES By To OTHER STATIONS f* 20\ IIIIIIIII TELEPHONE MODEM BANK CONTROLCENTER INPUT/OUTPUT MODEM coNTRoLLER PRINTER 90 F/G COMPUTER f FRoMVEHICLE MIXER FILTER MIXER FILTER I I OUTPUT To f2 OSCILLATOR ADDERDETECTION FROM f SYSTEM VEHICLE 3 /04 /07 /09 FILTER /o2 f I MIXERFILTER MIXER m PAIENIEDJAII 30 I975 SHEET 3 OF VEHICLE 30 ms 30 ms FIRSTI COMPUTER-RELAY STN.

2 RELAY STN.- VEHICLE 3 VEHICLE I.-RELAY STN.

4 RELAY STN.-COMPUTER FIX NO. I

THIRD I COMPUTER-RELAY STN. 3 VEHICLE 3- RELAY STN. 4 RELAYSTN.-COMPUTER FIX NO. 3

'///////////// VEHICLEII) VEHICLE (2) VEHICLE (3) TELEPHONE DATA RADIOTRANSMISSION TELEPHONE DATA CHANNEL 2700 BPS CHANNELS I200 BPS 1200 BPSs COMPUTER TO Y VEH'CLE IDENTITY MESSAGE MASTER STATION r CODE E MASTERRELAY s VEH STATION TO VEHICLE ID 2 2:2 35 AND SLAVE STNS C CODEA; E 7 ME VEHICLE TO RELAY i gg'gg g RANG'NG STATIONS SIGNAL RELAY STATIONS MSGRANGE To COMPUTER Q DATA DATA VEHICLE COMMAND AND CONTROL SYSTEMREFERENCE TO RELATED CASES Application Ser. No. 19,190 filed Mar. 13,1970, of Joseph E. Bryden titled Visual Display System and U.S. Pat. No.3,633,169, issued Jan. 4, 1972, of William J. Bickford titled DemandAccess Digital Communications System are both assigned to the sameassignee as the present application and are hereby incorporated hereinby reference.

BACKGROUND AND SUMMARY OF THE INVENTION This invention relates to thecommand and control of a large number of mobile vehicles, for examplesurface vehicles, such as police and emergency vehicles, and rapidtransit buses, and more particularly to the communication with andlocation of such vehicles in high clutter signal environments which arecharacteristic of urban centers having tall buildings.

In conventional two-way voice communications systems, a mobile radiotelephone in each vehicle communicates with a central base station. Afew remote relay stations are used when necessary in weak signal areas.In such systems a typical radio signal channel has a limited capacityfor handling vehicles, which means that as the system expands additionalchannels are required which channels may not be available because of thefrequency allocation policies of the government coupled with highdemand. Additionally, receiver monitors may be required at the basestation for each channel and only manual rather than automatic positionlocation and schedule adherence is possible. Additionally, the presenceof such a large number of signal sources arriving at a central pointincreases the probability of signal blockage. Additionally, the drivershandling of the vehicle must be disturbed to operate the radio.

An additional problem inherent in single frequency systems of the priorart is that transmissions can not occur simultaneously between a relaystation and a vehicle, and the vehicle and the relay station; however,by utilizing the two frequency method of the present invention,simultaneous transmission of the ranging signal both ways between therelay station and the vehicle halves the time for measurement orcorrespondingly doubles the sampling rate of the system thereby allowingan increased number of vehicles to be serviced without a correspondingincrease in a time required for transmission to the vehicles. Toaccommodate both the location and communication functions in the controlsystems, a class of systems called distributed roadside systems has beenemployed. These systems require the installation of equipment, eitherburied in the road or adjacent to the road on a post, the function ofwhich is to establish the position of a nearby vehicle since theposition of the roadside equipment is known. Reference may be made toU.S. Pat. No. 2,597,517 to D.E. Nobel issued on May 20, 1952 and U.S.Pat. No. 2,790,071 issued to D.L. Gunn on Apr. 23, 1957.

Distributed roadside systems may be divided into two groups, the firstgroup requiring the vehicle to transmit its identity to the roadsideequipment by radio with the roadside equipment receiving this signal andretransmitting the vehicle identity by, for example, direct telephonelines to a control center; while the second group operates in a conversemanner. In this situation the roadside equipment transmits its equipmentidentity code to the vehicle by radio. The vehicle retransmits both thecode of the roadside equipment and its vehicle identity by radiodirectly to a common control station. In both groups the accuracy oflocation of vehicles is directly related to the number and location ofthe roadside equipments. The more accurate the location, the moreroadside equipments are needed. As an additional disadvantage,flexability is limited because the vehicle must pass close to theroadside equipment in order to be detected.

The prior art also discloses a number of vehicle location systems basedupon triangulation from a number of known points. Attention is directedto U.S. Pat. No. 2,470,787 issued to P.W. Nosker on May 24, 1949relating to a system for determining the position or path of objects inspace. This system uses a plurality of ranging stations for phaseranging upon an airborne vehicle equipped with a transponder. No mentionis made of treating the effects of multipath caused by the CW wavebouncing back from the ionsphere.

In the contemporary art phase ranging of a vehicle in space has takenthe form of high frequency highly directed antennas and propogationpatterns. Additionally, the problems of multipath are in part avoided byusing directive antennas and by pointing them skywards. In this regard,reference is made to the Institute of Radio Engineers Transactions onAntennas and Propogation, October 1955 on pages -192 in an articleentitled Multipath Phase Errors in CW FM Tracking Systems by T.E.Solenberger.

Multipath effects are greatly reduced in the present system due to thediversity combination technique utilized with the dual frequency systemof the present invention. This technique is described in detail in U.S.Pat. No. 3,471,788 to WJ. Bickford et al., and is assigned to the sameassignee as the present invention.

It is therefore an object of this invention to provide a dual frequencyranging system for use with mobile vehicles in which phase stableoscillators are not required either in the vehicles or at the relaystation.

It is an additional object of this invention to provide a dual frequencyranging system in which interrogation signals are sent to a vehicle atone frequency and the response from the vehicle is at another frequency.

It is an additional object of this invention to provide a system forcommand and control of a fleet of mobile surface vehicles in a highmultipath signal environment especially in urban areas.

It is an additional object of this invention to provide for the locationand communication functions between and among vehicles and a remotestation utilizing similar equipment by means of single transmissionsfrom the vehicle.

It is yet an additional object of this invention to provide a system forthe efficient utilization of the limited bandwidth available to mobilevehicle voice radio communication channels. Relately, it is desired thatthe system permit vehicle location to be within a high order ofaccuracy.

It is yet another object of this invention to utilize digital coding inthe ascertainment of vehicle location and communication on a repetitiveor cyclical basis in a vehicle fleet expandable to several thousandunits.

The aforementioned objects and advantages are satisfied in an embodimentcomprising a common control arrangement and a plurality of remote relaystations, each station being capable of signal transmission andreception or reception only, and including means for communication withthe control arrangement and the plurality of transponders located incorresponding vehicles, each transponder being operable upon receipt ofa suitable coded digital signal transmitted from remote stations.

Continuous location and coded digital communications using a standardUHF radio channel are provided in which location is derived from aseries of phase range difference measurements with respect to thevehicles and relay stations. The control center transmits a digitalsignal containing a vehicle identification code, and a coded digitalmessage if one is required to be transmitted to the relay stationselected as a master station via a telephone data channel. The masterstation then transmits the digital signal together with a range tone asmodulation on the UHF radio carrier. The radio signal is received by allrelay stations and vehicles within range of the master station; however,only the vehicle identified by the particular code transponds thereceived range tone as modulation on the other frequency of the sameduplex radio channel. The transponded tone is compared in phase with themaster station and at those relay stations receiving it with theoriginal range tones transmitted by the master station.

The phase difference measurements are then transmitted to the controlcenter computer via the relay stations telephone data channels wherethey are processed and the vehicle location determined. The relaystations are arranged in an approximately square matrix across the areaof operation with an interstation spacing, for example, of 3 miles inthe dense downtown central area and 7 miles over the remainder of thearea. This arrangement provides a range measurement for each vehiclelocation by at least four relay stations with a resultant locationaccuracy within 600 feet with 95 percent probability of correctness.More measurements may be conducted when necessary in an emergency toprovide for an accuracy of location within 300 feet with 95 percentprobability of accuracy.

Real time data is provided to dispatchers via cathode ray tube displayconsoles such as the display described in the Bryden application, andtheir decisions, commands, or requests for direct communication with thevehicles may be made through the keyboard of the same console. Dataprovided by the vehicle fleets can be stored, processed and operatedupon by the individual control center computers for a large variety ofsystem operation requirements which are necessary for the effectiveoperation of a large fleet.

The invention contemplates means at one of the remote relay stationsunder control of the common control arrangement for transmitting anomnidirectional signal with coded indicia modulated thereon, means atselected transponders responsive only to the coded indicia forgenerating an omnidirectional coded reply signal after receipt of thetransmitted signal and receiving means at the remote relay station fordetecting and decoding the original transmitted signal and the replysignal and further including means for communicating the decoded replysignal to the common control arrangement.

Specifically, the ranging system of the present invention employs a fullduplex radio channel having two radio frequency channels spaced aboutfive megacycles apart. Of course, the frequency channels spacing may beas wide or as narrow as desired. The frequency spacing is wide enough toavoid interference between the two signal frequencies. The master relaystation uses one of these frequencies, for example,f and transmits dataand ranging tone signals modulated thereon to the vehicle and to thecorresponding relay stations. That vehicle which is selected by aspecific identification code signal responds by immediatelyretransmitting the same tone or ranging signal back to the relay stationmodulated on a second carrier frequency, for example, f Thus each relaystation and the master station, which may be one of the relay stationsin the computer selected group, receives two identical tone signalswhose phase delays are related to the distances between each station andthe location of the signal source. These phase difference measurementsare then coded and sent via telephone link to a central computer whichmay be at the master relay station or at a central location. Thecomputer then calculates the vehicle best estimate location and sendsthe results to output indicating and storage devices such as digitaldisplays and alphanumeric printouts. Thus, a system is provided wherebyvehicle information and location is centralized and accessible by meansof signals received at a plurality of relay stations from the pluralityof vehicles, such that the central location is aware of the location ofany vehicle at any given time. Because the vehicle transmissions at fonce received at the plurality of relay stations are relayed to thecentral location and the central computer by telephone links, there canbe no interference between the signals from the various relay stations,hence the vehicle location may be calculated by means of an algorithmcontaining the various phase differences of the modulation at the secondfrequency and the modulation at the first transmitted frequency at eachof the relay stations as will be explained.

The control center which has a central computer with associatedperipheral equipment such as visual displays and hard copy printersconnected to the relay stations by full duplex telephone data channels,automatically controls the total system on a sequential basis at a rateof, for example, one vehicle communication location every thirtymillseconds. It decides the order in which the vehicles are to beinterrogated and selects the best group of relay stations, usually thoseclosest to the vehicle, on which location and communication operationswill be performed. If the approximate location of the vehicle beinginterrogated is unknown, the computer may systematically search theentire urban area until the location is established. Successive locationdeterminations of the vehicle can then be performed in intervalssufficiently small that the computer can retain the approximate locationin its storage and the selection of the appropriate group of relaystations can be made without again going through a searching mode.

The ineffective voice communication use of already scarce radio channelsis alleviated by the provision of an integrated coded digitalcommunication and phase ranging location system utilizing only a duplexUHF radio channel. The basic system can locate and communicate with upto 2,000 vehicles per minute, however, should a higher rate be desiredadditional channels can be utilized together with additionaltransmitter-receivers at the relay stations.

BRIEF DESCRIPTION OF THE DRAWINGS Further objects and advantages of theinvention will become apparent from the following specifications takenin connection with the accompanying drawings, wherein like referencecharacters identify parts of like function throughout the differentviews thereof.

FIG. 1 is a block diagram of a communication and location system inaccordance with the present invention which is illustrative of alocation operation.

FIG. 2 is a block diagram of a specific embodiment of the presentinvention.

FIG. 3 is a block diagram of the predetection combination techniqueutilized in the present invention.

FIG. 4 illustrates the general timing sequence of the present invention.

FIG. 5 is illustrative of the composition, sequencing and timing offormatted digital messages transmitted between the system elements ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, thereis shown a vehicle control ranging and communication system inaccordance with the present invention wherein the location operation isillustrated. The control center shown generally at contains a centralcomputer 11, such as Raytheon Model 704, which transmits a messagecontaining the identity code of the particular vehicle with whichcommunication is required or desired. Each vehicle in a fleet ofvehicles, such as transmit buses, police cars, or taxi cabs shown as 12,13, and 14 respectively, has a particular and unique identity codecomprised of digits to which a transponder in that particular vehicleand only the transponder in that particular vehicle responds.

The transponder (not shown) is activated sequentially along with theother transponders in other vehicles in their respective fleets oftransit buses, police cars and/or taxis or any other vehicles which maybe present in the system.

The control center 11 transmits a digitally coded message to a masterrelay station 15 which is identical to slave relay stations l6, l7 and18 except for the presence of a transmitter as will be explained. Thismessage is transmitted via telephone land lines which, for example, maycomprise 1,200 bit per second twoway telephone lines 19 over which datamay be exchanged between the computer and the master relay stationthrough telephone modem bank 20 which is controlled by an input outputmodem controller 21 for buffering data into and out of computer 11. Thetelephone modem bank 20 and modem controller 21 are of well knowndesign, for example, the modem controller may be similar to that shownin the copending application of William J. Bickford. The modemsthemselves, representatively shown at 20, are of standard manufacturesuch as made by Western Electric, Milgo Electronics, or Rixon Corp. Oncethis message is received over line 19 at the master relay station 15, itis transmitted with a ranging tone modulated thereon which in thepresent embodiment may preferrably be 2,700 hertz as frequencymodulation on an RF carrier of frequency fl in the UHF region, forexample, 460 megahertz.

This transmitted signal is received by all relay stations and vehicleswithin range of the master station. In the embodiment shown in FIG. 1these are relay stations 16,17 and 18 and vehicles 12, 13 and 14.However, only the vehicle whose identity code was transmitted willretransmit (transpond the 2,700 hertz ranging time received from themaster station). This retransmission occurs as modulation on the otherRF carrier at frequency f of the allocated duplex UHF channel where f isideally approximately 5 megahertz displaced in frequency from f,;however, of course, the frequency displacement maybe other than 5megahertz. The signal transponded from the vehicle is received at themaster relay station and the other relay stations in the area.

A phase measurement is made at the master relay station 15 and each ofthe relay stations 16, 17 and 18 within range, with the vehicle rangedetermined by the phase difference between the ranging tone receivedfrom the master station and that received from the vehicles on theirrespective frequencies, f and f These phase differences are proportionalto the difference of path length traveled by the two transmissions.Since the relay station locations are known to the computer the positionof the vehicle can be determined in accordance with the algorithm to bedescribed.

The reply signal received from he vehicles is a digitally coded signalwhich is substantially omnidirectionally retransmitted within apredetermined time after receipt of the transmitted signal at thefrequency of the associated duplex radio channel. Each reply signal at fincludes coded data and a ranging signal modulated on f Asynchronization signal may or may not be transmitted from the vehicle.The composition of the individual coded signals is more completelydescribed with reference to FIGS. 4 and S in which figures the spacingand timing of these replay signals is also illustrated.

A number of relay stations receive the original transmission as well asthe transponded signal. .Consequently, when the reply signals atfrequency f from the vehicles are received at the stations the relativephase of the ranging signals may be measured and the measurementtransmitted back to the common control center 10. This relative phasemeasurement is of course related to the distance between the remoterelay station and the vehicle transponder.

The relative phase measured is perturbed by a number of phenomenon, themajor one being that of multiple signal reflections from buildings andother objects in a major city. The relative phase, as previouslydescribed, is measured at each of number of relay stations and whenthese measurements are incorporated into a location computation usingthe algorithm to be described which is based on inverse hyperbolicminimum variance principles, the vehicle location is described. Animportant attribute of this algorithm it that it results in thecancelation of mean relative phase errors due to multiple pathreflections, transponder phase shift, noise received at and generated bythe transponder, and at the relay stations. Thus, the algorithm enablestransponder and relay stations phase errors to cancel out, and therebypermits the use of less expensive nonphase stable equipment.

The equations suitable for an inverse hypebolic algorithm of the typewith which non-phase stable vehicle transceivers may be used will now bedescribed.

Considering M as a master relay station, S S and S as slave relaystations and V as the vehicle, qb is the phase difference measurementmade at any of the relay stations S S and S between two received tones.

The phase bias, is the deviation from phase delay through the vehicletransceiver at a 2,700 hertz frequency.

represents the multipath induced phase error of a path XY which path,for example, may be MS the distance between the master relay station andone of the slave relay stations. I" is the equivalent phase relay of a2,700 hertz tone having range XY.

4),, 2MV+ 6+ fi,,,,,+ [3

MV+ VS MS, (8 +BMv) Bvs, B ls.

(# MV+ VS MS, (8 Buy) Br BlS 5 MV+ VS MS (8 +13 My) Bys, ,8.us,

Let R V8,, R V8 R V5 R MV R MV= %(M BMV+BVM 4s. (M +5....) 4 BMV R VS 4:(MS BM.) Bl'sr' MV 8 [3 R VS (MS BMs.) fi t MV 8 Buy From the aboveequations, the following algorithm can be derived which can be solved atthe central computer of the system at the control center by iterationmethods to produce a vehicle location estimate.

R1- R2= (0s, "-43 MS1MS2) (Bus, "B1152,

(firm Bis,

R1 R3 (05', M51 M53) (/3.\1s, B.us,) (Bvs,B l'.s,)

From the above algorithm, it can be seen that the inverse hyperbolicequations can be derived at the central computer using non-phase stablereceivers and transmitters at the vehicles thereby greatly reducing theoverall system cost when many vehicles are involved, and also obviatingany serious calibration problems.

Small multipath induced phase errors in the master station to relaystation links are greatly reduced by the diversity reception techniqueof the present invention which may most effectively utilize predetectioncombining described with reference to FIG. 3. Although postdetectioncombining might also be employed in a diversity system, predetectioncombining results in superior performance in eliminating multipatheffects.

The rate of location is enhanced over that which can be achieved with asingle UHF radio channel by the use of a duplex UHF radio channel. Therelay station trans mits the UHF carrier signal at one of twofrequencies of the duplex UHF channel and the vehicle transpondertransmits its UHF carrier at the other frequency f of the duplexchannel. This allows the retransmission of the ranging signal from thevehicle simultaneously with reception of the ranging signal from therelay station without mutual interference which cannot be done in asingle frequency system. Additionally, the final phase measurements aremade at the relay station, and at at the master station, which may alsobe a relay station, and the transmission of these measurements viatelephone links to the control center results in no interference orconfusion between the respective phase measurements since eachmeasurement is transmitted on its own telephone link.

Referring now to FIG. 2, there is shown a vehicle ranging and controlsystem of the present invention in which a fleet of vehicles issequentially interrogated from a master relay station 15 under thecontrol of a central computer at a central station 10 with a pluralityof relay stations for receiving the vehicle response indicated at 16 and17. The central location includes a computer 11 selectively connectableto various files, memory and print out indicated generally at 12 and todigital displays 24 of the type disclosed in the patent application ofJoseph E. Bryden, previously incorporated herein.

The computer contains sequences of program order words and datarepresentative of the message to be formatted, and may further include aprogram for automatic rechecking and regulating the system.

Illustratively, computer 11 communicates with one or more of the remotestations 16 and 17 at the master station 15 and others not shown overcorresponding data links 30, 31, 32, 33, and 19 comprising telephonelines terminating in corresponding modems 36, 37 and 38 in the modembank 20. Of course, other modems, not shown, associated with other relaystations are not shown. The modems 36, 37 and 38 in turn terminate ininput-output modem controller 21 of the general type described in thepatent application of William J. Bickford, previously incorporatedherein. The modem controller interfaces computer 11 through appropriatedata buffers (not shown).

Control center 10 services a matrix of relay stations throughout a cityof interest situated at intervals of, for example, 3 to 7 miles apart,the exact spacing being dependent upon the topography of the area, thenoise environment and the availability of appropriate sites. Each relaystation is connected with the control center via the telephone landlines as previously described. Once the computer 11 selects a relaystation to act as master station for a sector, that station is thencommanded to the master mode via the telephone land line associated withthat particular relay station and requested to locate the specificvehicles known by the computer to be in the surrounding region for themaster station, which knowledge by the computer is based upon continuinglocation information since the entry of the vehicle into the system. Itis to be understood that in the embodiment shown only one relay stationis capable of activation into the master mode since to equip all relaystations with such a capability would involve the addition of atransmitter at the frequency f,', however, each relay station couldalternatively be equipped as a master station.

The actual message format of the message sent by the computer to masterstation 15 is shown in FIG. 5. This message comprises a synchronizationportion of three to five bits, a vehicle identity code of, for example,15 bits plus parity bits and the coded message of five bits and parity.Of course, other message lengths may be used.

The master station transmits series of signals as modulations on the RFcarrier, each containing a vehicle identification code and a rangingsignal for positional location. Each signal is received by all relaystations and vehicles within range. Signals transmitted by the masterstation each contain a particular vehicle identity code so that althoughmany vehicles may receive a signal only the specific vehicle beingaddressed for location will respond to any one particular signal.

The radio signal at frequency f,, the format of which is shown in FIG.is transmitted by the master station to the vehicle and has a formatsuch that synchronization bits, vehicle identity code and the codedmessage are present along with the ranging tone of, for example, 2,700hertz modulated onto the f carrier. The requisite operational commandsare sent via lines 19 from the computer through telephone modem 38 totelephone modem 40 at the master station which feeds a modem controller41 similar to modern controller 21 at the central station whichcontroller supplies the information from the central location through asignal processor 42 which serves to synchronize the transmission andreception of frequency f and the reception of frequency f fromtransmitter 43 for f and receiver 44 for f and diversity network 45 forf Antenna 47 transmits f from transmitter 43 while f is also coupled toreceive 44. Of course, a conventional duplexer and circulator may beused to provide isolation where desired, however, the frequencyseparation between f and f is such that a circulator is not required.

During routine location operation, all vehicles in the system arelocated on a rotational basis. The computer selects a master station tocover a set of sectors and transmits to that station via the telephonelines a series of message location requests for all specific vehicleswithin those sectors as derived from the computer memory filescontaining the previous locations. A typical matrix of master and slavestations, for example, may contain 25 relay stations, with each masterrelay station covering a minimum of four sectors or three slave stationsand the master itself. The next relay station in the grid of stationswould cover the adjacent four sections and so on thereby minimizingrelay station switching operation.

While the actual location operation is as described above, in the eventof a request for an emergency loca tion, which can originate from thecomputer as a result of an emergency message or from one of thedispatchers or by request from the vehicle itself, the routine locationoperation may be interrupted and a priority message transmitted to theappropriate relay station or master station for transmission of themessage and ranging tone to the vehicle for carrying out an emergencylocation of the vehicle; In the event that a vehicle wishes tocommunicate routinely with the control center, the operator will enterthe coded message he wishes to transmit into the vehicle transponder. Inthe case of priority or emergency messages to the vehicle, since it isnot desirable that the message be delayed even the short time to thenormal communication location period for that vehicle, a specialemergency priority period may be allocated within each 30 millisecondperiod allocated to location of each particular vehicle.

It is highly unlikely that a number of vehicles in emergency or prioritymessage situation will each transmit in the same emergency priority timeperiod which would entail more than one vehicle operator pushing anemergency or priority transmit button within the same fixed 30millisecond period and it is even more unlikely that more than onevehicle would be received by exactly the same relay stations. As long asonly one relay station receives the transmission of only one vehicle ofthose vehicles transmitting, the message is of course sent to thecomputer. In the unlikely event that two or more vehicles have eachtransmitted only to the same relay station and the message is garbledbecause of this, then no computer acknowledgement of reception will bereceived at the vehicle transponder and automatic retransmission willoccur until the emergency priority message has been acknowledged by thecomputer. This may be accomplished by a routine program. The actualdelay in transmission of a priority message when the original prioritytransmission is garbled, is dependent upon the number of vehicles beinglocated in its position in time with respect to its specific locationperiod. In the case of 500 vehicles being located, the maximum timedelay is seconds. For a 1,000 vehicles being located it is seconds andso on. The minimum delay is less than 30 milliseconds. Of course, ifadditional channels are used, the message delay may be correspondinglyreduced.

The relay stations operating in a relay mode receive both theinterrogating signal and the vehicle transmission at f and frespectively. The ranging tone modulated on frequency f transmitted fromthe vehicle'is received at relay station 16 via antennas and 51 coupledto diversity receivers 52 and 53 where, after predetection combining atpredetection combiner 54, the phase delayed ranging tone is fed tosignal processor 55 with the modulation on carrier f, which is receivedat antenna 56 and coupled through receiver 57 to signal processor. Thesephase delay modulations are fed through modern controller 58 to a phasemeter 59 where the actual phase measurement is made, coupled backthrough the controller, put into digital form and modulated onto a 1,200bit per second telephone channel at modem 60. Delay encoding as used inthe present system is well known in the art and, for example, isdescribed in the Proceedings of the IEEE, July 1969, pages l3, 14, tol3, l6, titled Delay Modulation.

Relay station 17 is identical to relay station 16 and provides theadditional phase measurement required for solution of the locationalgorithm along with the measurements provided by phase meters 59 ofstation 16 and 48 of station 15. As in station 16, frequency f, is

received at antenna 61 and coupled through receiver 64 to signalprocessor 67 where the ranging modulation is extracted together with thefrequency f ranging modulation which is inputted to the signal processorvia combiner 71 which is supplied with the f modulation from diversityreceivers 65 and 66 supplied by antennas 62 and 63 respectively. Thisphase information is coupled through controller 68 to the phase meter 69at which point phase measurements are made, is coupled back through thecontroller 68 to the telephone modem 70 and is transmitted at 1,200 bitsper second as modulation on telephone line 33 to telephone modem 36 atthe central station at which point this information is supplied to thecomputer 11 through modem controller 39 together with the phasemeasurements taken at the other relay stations in the sectors servicedby particular master and slave stations.

The phase measurement may be accomplished by counting a clock frequencywhile the signals received from the master station and the vehicle areof opposite polarity. This is the equivalent of an exclusive OR"function, and may be used rather than the well known zero crossing phasemeasurement method to reduce errors resulting from noise crossingsterminating the phase count. The phasemetic itself may be of well knownand conventional design.

Of course, voice communication between the control center and thevehicle can be established by means of a coded message requesting it.

A phase shifter, not shown, is included at each relay station receiverto remove any error due to finite processing or signal delay through thereceivers. The phase shifter allows the total phase delay through eachreceiver to be adjusted to a multiple of 180 of phase at 2,700 hertz sothat the relay station dual receivers of frequencies f, and f havematched delay. The possibility of an error due to slow variation ofphase delay through the receivers between adjustments may be reduced byperiodic computer requested calibrations which derive the difference inphase delay through the dual receivers. The phase meters present at therelay stations may be of well known and conventional design with a rangeof 180 of phase at 2,700 hertz to obviate the possibility of ambiguousrange readings due to multiples of 180 of phase which would requirecorrection by additional computer operation. As mentioned above, thephase difference between the ranging tones received at a relay stationfrom the master relay station and from the vehicle is measured by meansof a digital phase meter. Although 48 cycles of the 2,700 Hertz rangingtone are transmitted and transponded, only 32 cycles are needed for themeasurement. The redundent cycles insure against loss of tone cyclethrough fading, impulse noise, processing delay, etc. The count is thenstored. Since crossings two through zero phase occur at each cycle itcan be seen that 32 cycles will give 64 individual counts. Phasemeasurement operation is designed to insure 64 actual measurements whichare added together and divided by 64 to produce an average phasedifference measurement.

This count is then transmitted to the computer via the relay station tocontrol center telephone line where it is used in the locationprocessing. Received carrier level and carrier noise ratio credenceindications of the received signals may also be sent to the computer aspart of the message so that the measurement can be rejected or correctedfor signal level effect.

Referring now to FIG. 3, the diversity combining predetectioncombination technique of the present invention is illustrated. The rangemeasurement accuracy is substantially improved by elimination ofmultipath effects by using diversity reception at the received signalfrequency f from the vehicle, which may include either predetection orpostdetection confirmation. The predetection combination technique beingmost effective is illustrated. Space or polarization diversity may beemployed. The reason for the range accuracy improvement is the reducedmodulation phase distortion achieved when two essentially independentsignals are combined. The ranging information modulated on frequency ffrom the vehicles is received at two different antennas 101 and l02nandmay be amplified before being fed to conventional mixers 103 and 104which may be heterodyning devices. A common local oscillator 105 isconnected to mixers 103 and 104 which produces pairs of intermediatefrequency signals from each mixer. Frequency selective filter 106, whichcan be a well known RLC filter is tuned to the difference frequencyoutput of mixer 103 and a similar frequency selective filter 107 istuned to the difference frequency output of mixer 104. It should beunderstood that filters 106 and 107 can be tuned to the sum frequencyoutput of mixers 103 and 104 and the remainder of each channel adaptedto run on the sum frequency. These filters improve the rejection ofunwanted mixer frequencies. The output of filters 106 and 107 is fed(after additional amplification and filtering if necessary) to mixers108 and 109, respectively which are linear mixers. The other inputsignal for mixers 108 and 109 in the received space diversity signals oncarrier f The output of mixer 108 is fed to a conventional RLC bandpassfilter 110 and the output of mixer 109 is fed to a conventional RLCbandpass filter 111. The output of filters 110 and 111 is fed to aconventional adder 112 which provides a signal proportional to thevector sum of the input signals. The predetection combination techniqueis described in greater detail in the beforementioned patent to W..1.Bickford.

Referring now to FIGS. 4 and 5, the system operation timing andoverlapping message technique for a single location of a vehicleincluding examples of the message format used to define thecommunication between system elements is shown.

Computer 11 in communicating with any of the remote relay stations asdescribed above, transmits a message over a telephone data channelhaving an approximate information capacity of 1,200 bits per secondalthough, of course, other telephone or data transmission lines ofhigher bit rates may be employed where necessary. The time required fora single location operation may be for example, 90 milliseconds dividedinto three segments of 30 milliseconds each as illustrated in FIG. 5.

During the first 30 milliseconds, central computer 11 transmits overtelephone lines 30 through 35 and over other telephone lines to otherrelay and master stations not shown, a message formatted as illustratedin FIG. 5. This message is a command to locate specific vehicles knownby the computer to be in the surrounding region for the master station.The synchronization bits, vehicle identity code with parity check, andthe message as such are sent.

Once having received this message via the telephone land line the masterstation transmits the message portion of the digitally coded signal asfrequency modulation together with a 2,700 hertz ranging tone on the RFcarrier at frequency f during the second millisecond interval of the 90millisecond period. This signal is received by the slave stations suchas 16 and 17 and any vehicles within range. The vehicle whose identitycode is contained in the message will transpond the tone as a frequencymodulation on the RF carrier f which is then received by the master andslave stations within range. The measurements of phase differencebetween the tones received from the master station and from the vehicleare made during the second 30 millisecond period. A short coded digitalmessage is also transmitted by the vehicle prior to the ranging toneduring this second 30 millisecond period, the message consisting of fivedata bits and one parity bit although, of course, other message andparity bit lengths may be employed.

During the third 30 millisecond period a message is transmitted by eachrelay station to the computer via its station-to-control centertelephone line and modem interface. This message contains the data fromthe measurements of phase difference as well as synchronization andparity bits in the coded digital message. It is important that thetransmission of the coded digital messages by the vehicle is arranged tooccur only in the second 30 millisecond period of a 90 millisecondlocation period. The coded digital message transmitted by the vehicle oncarrier frequency f can, as previously discussed, be either routine orpriority.

Referring now to FIG. 4, the time interleaving between the transmissionsto and from the various relay stations and to and from the vehicle isillustrated. As previously described, the vehicles respond on afrequency other than that which interrogation is made to the vehicles.This allows a vehicle response to occur simultaneously with aninterrogating signal to either another vehicle or to the vehicle whichis responding.

ln single frequency systems of the prior art simultaneous retransmissionwas impossible because there was no way of distinguishing theinterrogating signal from the reply signal; however, the dual frequencytime interleaving technique of the present invention allows suchsimultaneous transmissions and replays. By way of example, an arbitrary150 millisecond transmission period has been chosen in which theinterleaving of the transmission and response between the computer andthree relay stations is described. During the first 30 millisecondperiod the central computer transmits a first cyclic message to a relaystation for a first vehicle via telephone data channels.

During the next 30 milliseconds the relay station transmits via a radiolink at frequency f the interrogating signal to the first vehicle.During this same 30 millisecond period the transponder in the vehiclereplies to the relay station after the synchronization and vehicleidentity portion of the message are received by the vehicle and thecorresponding message and 2,700 hertz ranging tone is transmitted asmodulation on frequency f Also during this same 30 millisecond periodthe computer is transmitting a second interrogating message to the relaystation via the telephone link. Thus, during the second 30 millisecondperiod three messages, two of which are interrogatory and one of whichis a reply are transmitted, one by telephone link and two by radio.

During the third 30 millisecond period, four messages are simultaneouslytransmitted. The relay station transmits a communication and rangingsignal to the second vehicle at frequency f,, the relay station isresponding to the computer telephone data link with the phase andcommunication information from vehicle one, vehicle two is responding tothe relay station at frequency f with ranging phase modulation at 2,700hertz modulated thereon and the computer is transmitting via thetelephone link the communication and ranging interrogation message forthe third vehicle.

During the fourth 30 millisecond interval, that is, between andmilliseconds after the initiation of transmission, the relay station isrelaying the interrogating signal at frequency f to the third vehicle,the relay station is relaying via the telephone link the phaseinformation from the second vehicle, and the third vehicle istransponding at frequency f tothe relay station.

During the fifth 30 millisecond interval, that is between 120 and l50milliseconds, the relay station is relaying via the telephone link theranging and message information from the third vehicle to the centralcomputer. At the end of the 90, 120 and millisecond intervals, locationfixes are obtained upon vehicles one, two and three respectively. Thesefixes are indicated in FIG. 4 by the triangular symbols.

Of course, provision may be made for voice communication between thevehicle and any of the relay stations, the master station or the controlstation, if so desired. In FIG. 2, a typical voice communicationarrangement is illustrated between the control station and the masterrelay station in which voice modems 80 are coupled via telephone linksto modems 81 and 82 at the master station, which in turn may be used toprovide communication to transmitter and receiver pairs 83 and 84respectively operating at frequencies other than those previouslydescribed with respect to the ranging system, for example, f and fConventional duplexers 85 and 86 coupled to antennas 87 and 88 providethe-necessary communication capability, however, separate antennas mayof course be used in place of duplexers if desired. Conventionaltelephone voice switching circuitry 89 is used to route voicecommunication both to the dispatcher and through suitable buffercircuitry (not shown) to and from computer 11 if desired.

The vehicle transponders may be of well known and conventional designfor receiving coded digital signals of the type described with referenceto the IEEE article.

While particular embodiments of the invention have been shown anddescribed, various modifications thereof will be apparent to thoseskilled in the art and therefore it is not intended that the inventionbe limited to the disclosed embodiments or to details thereof anddepartures may be made therefrom within the spirit and scope of theinvention as defined in the appended claims.

What is claimed is:

1. In combination:

means for transmission of a substantially continuous wave signal of afirst frequency from a first station at a first location;

at least one means at a second location for receiving said firstfrequency substantially continuous wave signal and for transmitting asignal at a second frequency;

a plurality of additional stations at other locations for receiving saidfirst and second frequency signals and for generating a measurementindicative of the time difference between said first and secondfrequency signals; and

means for receiving said measurements from said plurality of additionalstations for determining the position of said means at said secondlocation.

2. A combination in accordance with claim 1 wherein at least one of saidreceiving means includes a predetection signal combination network.

3. A combination in accordance with claim 1 wherein at least one of saidreceiving means includes a postdetection signal combination network.

4. In combination:

means for transmission of a substantially continuous wave signal of afirst frequency from a first station at a first location, at least onemeans at a second location for receiving said first frequency signal andfor transmitting a substantially continuous wave signal at a secondfrequency in response thereto;

a plurality of additional stations at other locations for receiving saidfirst and second frequency signals and for generating a phasemeasurement therefrom; and

computing means for receiving said phase measurements from saidplurality of additional stations for determining the position of saidtransponder.

5. A combination in accordance with claim 4 wherein said secondfrequency signal is received at said first station and means at saidfirst station for generating a phase measurement from said first andsecond frequency signals which is supplied to said computing means withthe phase measurements from said plurality of additional stations fordetermining the position of said transponder.

6. In combination:

means for transmission of a signal carrier of a first frequency from afirst station at a first location;

means for frequency modulating said carrier at a predeterminedmodulation frequency;

at least one transponder means at a second location for receiving saidfirst frequency signal and for transmitting a signal at a secondfrequency in response thereto;

means at said second location for frequency modulating said firstfrequency signal at the same modulation frequency as said firstfrequency signal is modulated;

a plurality of additional stations at other locations for receiving saidfirst and second frequency carrier signals with said modulationfrequency modulated thereon and for generating a phase measurementbetween the two received modulation signals; and

computing means for receiving the phase measurement from said pluralityof additional stations for determining the position of said transponder.

7. A combination in accordance with claim 6 in whichf, andf are in theUHF region.

8. A phase ranging system comprising a master station, a plurality ofphase stations, first transmission means at said master station fortransmitting a modulated substantially continuous wave signal at a firstfrequency, transponder means at at least one mobile vehicle forreceiving said frequency signal and for transmitting a modulatedsubstantially continuous wave signal at a second frequency in responsethereto, means at said master station and at said slave stations forreceiving said first and second frequency modulated signals;

means at said master station and at said slave stations for phasecomparing the modulation on said first frequency and on said secondfrequency to obtain a plurality of distance measurements of the vehiclefrom the respective master and slave stations; and

means for transmitting the plurality of phase measurements to a centralcomputing means for determining the position of said mobile vehicle.

9. A phase ranging system comprising a master station, a plurality ofphase stations, first transmission means at said master station fortransmitting a modulated signal at a first frequency, transponder meansat at least one mobile vehicle for receiving said first frequency signaland for transmitting a modulated signal at a second frequency inresponse thereto, means at said master station and at said slavestations for receiving said first and second frequency modulatedsignals;

means at said master station and at said slave stations for phasecomparing the modulation on said first frequency and on said secondfrequency to obtain a plurality of distance measurements of the vehiclefrom the respective master and slave stations; means for transmittingthe plurality of phase measurements to a central computing means fordetermining the position of said mobile vehicle; and said means fortransmission of said phase measurements to said central control meanscomprising telephone lines.

10. In combination; a ranging and communication system comprising:

a central processor;

a plurality of remote relay stations;

at least one vehicle equipped with a transponder;

means at at least one of said remote relay stations for transmitting anomnidirectional signal having coded indicia and a ranging signalmodulated thereon; means at said transponder responsive only to saidcoded indicia for generating a coded reply signal including a rangingsignal within a predetermined time after receipt of said transmittedsignal;

receiving means at each of said plurality of remote relay stations fordetecting and decoding said signal transmitted from said transponder andsaid omnidirectional signal;

means at each of said plurality of remote relay stations for generatinga measurement indicative of the time difference between the transmissionfrom said relay station and the transmission from said transponder;

means for transmitting said detected and decoded signals from saidplurality of relay stations to the central processor.

11. A combination phase ranging and communication system in accordancewith claim wherein said omnidirectional signal transmitted from at leastone relay station includes a carrier at a first frequency with a rangetone modulated thereon and said transponder signal from said vehiclegenerated in response to the receipt of said omnidirectional signalcomprises a carrier at a second frequency with a ranging tone modulatedthereon.

12. A combination phase ranging and communication system in accordancewith claim 11 wherein the portion of said first and said secondmodulated signals containing the range tone modulated on said respectivefirst and second frequency signals is transmitted simultaneously fromsaid remote relay station and from said vehicle.

13. A combination phase ranging and communication system in accordancewith claim 11 wherein said omnidirectional signal additionally containsa vehicle identification code modulated thereon such that only thetransponder on the particular vehicle for which said identity code isintended will transpond in response to said omnidirectional signal.

14. A combination phase ranging and communication system in accordancewith claim 13 wherein said second frequency signal containing the rangetone modulated thereon is transmitted simultaneously with said firstomnidirectional signal having said range tone modulated thereon.

15. A combination phase ranging and communication system in accordancewith claim 13 further comprising phase measuring means at each of saidremote relay stations for measuring the phase difference between theranging tones modulated on said first and second frequency signals toderive positional information as to the location of said vehicle; and

means for transmission of said plurality of phase measurements to saidcentral processor for computation of the vehicle location.

16. A combination phase ranging and communication system in accordancewith claim 15 wherein said means for transmission of said phaseinformation from the plurality of relay stations to the centralprocessor is a plurality of telephone data channels and furtherincluding a telephone data channel for transmission of control signalsfrom the central processor to said remote relay station transmittingsaid omnidirectional signal.

17 A combination phase ranging and communication system in accordancewith claim 16 wherein said central processor comprises a digitalcomputer.

18. A phase ranging system for command and control of a fleet of mobilevehicles in a high multipath signal clutter environment such as a citycomprising:

a plurality of remote relay stations for transmission of anomnidirectional frequency modulated substantially continuous wavecarrier of a first frequency;

a plurality of transponders located at different vehicles;

each of said transponders being operable upon receipt of said firstfrequency signal for activating means for transmitting a frequencymodulated substantially continuous wave carrier at a second frequency;and

means at said plurality of remote relay stations for receiving saidfirst and second frequency modulated signals and for deriving a phasemeasurement therefrom indicative of the location of said transponders.

19. A phase ranging system for command and control of a fleet of mobilevehicles in a high multipath signal environment such as a city inaccordance with claim 18 further including diversity reception means fordiversity reception of said transponded signals, including two receiversfor receiving said second frequency signal; and a predetection combinerfed with the detected modulation on said second frequency signal fromboth of said receivers.

20. A phase ranging system for command and control of a fleet of mobilevehicles in a high multipath signal clutter environment such as a citycomprising:

a plurality of remote relay stations for transmission of anomnidirectional frequency modulated carrier of a first frequency;

a plurality of transponders located at different vehicles;

each of said transponders being operable upon receipt of said firstfrequency signal for activating means for transmitting a frequencymodulated carrier at a second frequency;

means at said plurality of remote relay stations for receiving saidfirst and second frequency modulated signals and for deriving a phasemeasurement therefrom indicative of the location of said transponders;and

said frequency modulated carrier transmitted from said plurality ofremote relay stations having modulated thereon a formatted messagehaving a synchronization portion, a vehicle code identification portion,a coded data portion, and a ranging signal such that only the particularvehicle transponder containing a corresponding vehicle identificationcode will transpond in response to said transmission.

21. A system in accordance with claim 20 in which said formatted messageis a digital code modulated onto said carrier.

22. A system in accordance with claim 21 in which said second frequencymodulated carrier transmitted from said transponder contains a codeddigital message and a ranging tone modulated thereon is transmittedsimultaneously with the transmission of the portion of said formattedmessage modulated on said first frequency carrier that contains thedigitally coded message and the ranging tone from said relay station.

23. A phase ranging system in accordance with claim 22 wherein saidvehicle code identification portion of said first frequency signal keyson the transponder responsive to that particular vehicle identity codeto transpond simultaneously with the remainder of the message at saidfirst frequency, said transponder frequency being at said secondfrequency.

24. A phase ranging system for use with a fleet of mobile surfacevehicles comprising:

a central processor including a digital computer;

means at each of said relay stations for measuring the phase differencebetween said first and second frequency signals for determining theposition of said transponder;

wherein the improvement comprises:

the transmitting and receiving means at said plurality of relay stationsand said vehicle transponder comprising non-phase stable components suchthat said phase measurements are independent of phase variationsinherent in said receivers and transmitters at said plurality of relaystations and at said transponder in the vehicle.

1. In combination: means for transmission of a substantially continuouswave signal of a first frequency from a first station at a firstlocation; at least one means at a second location for receiving saidfirst frequency substantially continuous wave signal and fortransmitting a signal at a second frequency; a plurality of additionalstations at other locations for receiving said first and secondfrequency signals and for generating a measurement indicative of thetime difference between said first and second frequency signals; andmeans for receiving said measurements from said plurality of additionalstations for determining the position of said means at said secondlocation.
 1. In combination: means for transmission of a substantiallycontinuous wave signal of a first frequency from a first station at afirst location; at least one means at a second location for receivingsaid first frequency substantially continuous wave signal and fortransmitting a signal at a second frequency; a plurality of additionalstations at other locations for receiving said first and secondfrequency signals and for generating a measurement indicative of thetime difference between said first and second frequency signals; andmeans for receiving said measurements from said plurality of additionalstations for determining the position of said means at said secondlocation.
 2. A combination in accordance with claim 1 wherein at leastone of said receiving means includes a predetection signal combinationnetwork.
 3. A combination in accordance with claim 1 wherein at leastone of said receiving means includes a postdetection signal combinationnetwork.
 4. In combination: means for transmission of a substantiallycontinuous wave signal of a first frequency from a first station at afirst location, at least one means at a second location for receivingsaid first frequency signal and for transmitting a substantiallycontinuous wave signal at a second frequency in response thereto; aplurality of additional stations at other locations for receiving saidfirst and second frequency signals and for generating a phasemeasurement therefrom; and computing means for receiving said phasemeasurements from said plurality of additional stations for determiningthe position of said transponder.
 5. A combination in accordance withclaim 4 wherein said second frequency signal is received at said firststation and means at said first station for generating a phasemeasurement from said first and second frequency signals which issupplied to said computing means with the phase measurements from saidplurality of additional stations for determining the position of saidtransponder.
 6. In combination: means for transmission of a signalcarrier of a first frequency from a first station at a first location;means for frequency modulating said carrier at a predeterminedmodulation frequency; at least one transponder means at a secondlocation for receiving said first frequency signal and for transmittinga signal at a second frequency in response thereto; means at said secondlocation for frequency modulating said first frequency signal at thesame modulation frequency as said first frequency signal is modulated; aplurality of additional stations at other locations for receiving saidfirst and second frequency carrier signals with said modulationfrequency modulated thereon and for generating a phase measurementbetween the two received modulation signals; and computing means forreceiving the phase measurement from said plurality of additionalstations for determining the position of said transponder.
 7. Acombination in accordance with claim 6 in which f1 and f2 are in the UHFregion.
 8. A phase ranging system comprising a master station, aplurality of phase stations, first transmission means at said masterstation for transmitting a modulated substantially continuous wavesignal at a first frequency, transponder means at at least one mobilevehicle for receiving said frequency signal and for transmitting amodulated substantially continuous wave signal at a second frequency inresponse thereto, means at said master station and at said slavestations for receiving said first and second frequency modulatedsignals; means at said master station and at said slave stations forphase comparing the modulation on said first frequency and on saidsecond frequency to obtain a plurality of distance measurements of thevehicle from the respective master and slave stations; and means fortransmitting the plurality of phase measurements to a central computingmeans for determining the position of said mobile vehicle.
 9. A phaseranging system comprising a master station, a plurality of phasestations, first transmission means at said master station fortransmitting a modulated signal at a first frequency, transponder meansat at least one mobile vehicle for receiving said first frequency signaland for transmitting a modulated signal at a second frequency inresponse thereto, means at said master station and at said slavestations for receiving said first and second frequency modulatedsignals; means at said master station and at said slave stations forphase comparing the modulation on said first frequency and on saidsecond frequency to obtain a plurality of distance measurements of thevehicle from the respective master and slave stations; means fortransmitting the plurality of phase measurements to a central computingmeans for determining the position of said mobile vehicle; and saidmeans for transmission of said phase measurements to said centralcontrol means comprisinG telephone lines.
 10. In combination, a rangingand communication system comprising: a central processor; a plurality ofremote relay stations; at least one vehicle equipped with a transponder;means at at least one of said remote relay stations for transmitting anomnidirectional signal having coded indicia and a ranging signalmodulated thereon; means at said transponder responsive only to saidcoded indicia for generating a coded reply signal including a rangingsignal within a predetermined time after receipt of said transmittedsignal; receiving means at each of said plurality of remote relaystations for detecting and decoding said signal transmitted from saidtransponder and said omnidirectional signal; means at each of saidplurality of remote relay stations for generating a measurementindicative of the time difference between the transmission from saidrelay station and the transmission from said transponder; means fortransmitting said detected and decoded signals from said plurality ofrelay stations to the central processor.
 11. A combination phase rangingand communication system in accordance with claim 10 wherein saidomnidirectional signal transmitted from at least one relay stationincludes a carrier at a first frequency with a range tone modulatedthereon and said transponder signal from said vehicle generated inresponse to the receipt of said omnidirectional signal comprises acarrier at a second frequency with a ranging tone modulated thereon. 12.A combination phase ranging and communication system in accordance withclaim 11 wherein the portion of said first and said second modulatedsignals containing the range tone modulated on said respective first andsecond frequency signals is transmitted simultaneously from said remoterelay station and from said vehicle.
 13. A combination phase ranging andcommunication system in accordance with claim 11 wherein saidomnidirectional signal additionally contains a vehicle identificationcode modulated thereon such that only the transponder on the particularvehicle for which said identity code is intended will transpond inresponse to said omnidirectional signal.
 14. A combination phase rangingand communication system in accordance with claim 13 wherein said secondfrequency signal containing the range tone modulated thereon istransmitted simultaneously with said first omnidirectional signal havingsaid range tone modulated thereon.
 15. A combination phase ranging andcommunication system in accordance with claim 13 further comprisingphase measuring means at each of said remote relay stations formeasuring the phase difference between the ranging tones modulated onsaid first and second frequency signals to derive positional informationas to the location of said vehicle; and means for transmission of saidplurality of phase measurements to said central processor forcomputation of the vehicle location.
 16. A combination phase ranging andcommunication system in accordance with claim 15 wherein said means fortransmission of said phase information from the plurality of relaystations to the central processor is a plurality of telephone datachannels and further including a telephone data channel for transmissionof control signals from the central processor to said remote relaystation transmitting said omnidirectional signal. 17 A combination phaseranging and communication system in accordance with claim 16 whereinsaid central processor comprises a digital computer.
 18. A phase rangingsystem for command and control of a fleet of mobile vehicles in a highmultipath signal clutter environment such as a city comprising: aplurality of remote relay stations for transmission of anomnidirectional frequency modulated substantially continuous wavecarrier of a first frequency; a plurality of transponders located atdifferent vehicles; each of said transponders being operable uponreceipt of said first frequency signal for acTivating means fortransmitting a frequency modulated substantially continuous wave carrierat a second frequency; and means at said plurality of remote relaystations for receiving said first and second frequency modulated signalsand for deriving a phase measurement therefrom indicative of thelocation of said transponders.
 19. A phase ranging system for commandand control of a fleet of mobile vehicles in a high multipath signalenvironment such as a city in accordance with claim 18 further includingdiversity reception means for diversity reception of said transpondedsignals, including two receivers for receiving said second frequencysignal; and a predetection combiner fed with the detected modulation onsaid second frequency signal from both of said receivers.
 20. A phaseranging system for command and control of a fleet of mobile vehicles ina high multipath signal clutter environment such as a city comprising: aplurality of remote relay stations for transmission of anomnidirectional frequency modulated carrier of a first frequency; aplurality of transponders located at different vehicles; each of saidtransponders being operable upon receipt of said first frequency signalfor activating means for transmitting a frequency modulated carrier at asecond frequency; means at said plurality of remote relay stations forreceiving said first and second frequency modulated signals and forderiving a phase measurement therefrom indicative of the location ofsaid transponders; and said frequency modulated carrier transmitted fromsaid plurality of remote relay stations having modulated thereon aformatted message having a synchronization portion, a vehicle codeidentification portion, a coded data portion, and a ranging signal suchthat only the particular vehicle transponder containing a correspondingvehicle identification code will transpond in response to saidtransmission.
 21. A system in accordance with claim 20 in which saidformatted message is a digital code modulated onto said carrier.
 22. Asystem in accordance with claim 21 in which said second frequencymodulated carrier transmitted from said transponder contains a codeddigital message and a ranging tone modulated thereon is transmittedsimultaneously with the transmission of the portion of said formattedmessage modulated on said first frequency carrier that contains thedigitally coded message and the ranging tone from said relay station.23. A phase ranging system in accordance with claim 22 wherein saidvehicle code identification portion of said first frequency signal keyson the transponder responsive to that particular vehicle identity codeto transpond simultaneously with the remainder of the message at saidfirst frequency, said transponder frequency being at said secondfrequency.